Sir first thing is to remove CNG and run on Petrol if you can afford as the City |L13A IDSI does not like CNG at all.
Secondly inspect and change the plugs (all 8 plugs) if necessary, Denso K20 PRU 11 are stock, if you can afford then IK20 are good Iridium long life replacement.
Also change your air filter to a good paper one and tune your Timing advancer and CNG kit.
Regards